From the nineteenth century and almost until the end of World War II, the 37mm artillery caliber was universal for almost all warring and neutral countries, including the “inch” countries – the USA and Great Britain. The history of adopting such a caliber stems from the 1868 international declaration in St. Petersburg, which prohibited the use of high-explosive shells weighing less than 400 grams (just under one pound), which roughly corresponded to the 37mm caliber.
And although the declaration’s decisions were effectively discarded during World War I, the 37mm gun caliber remained popular and was widely used by most countries in both world wars across the broadest spectrum of applications – in tank and anti-tank, aircraft and anti-aircraft, infantry and naval guns. Even today, 37mm anti-aircraft guns are still produced in China by NORINCO.
The Legacy of the 37mm Caliber
In the interwar period, some powers, assessing that the 37mm caliber was no longer powerful enough, especially for tank and anti-tank roles, simply increased it to 47mm. However, for use in aviation and anti-aircraft artillery, the 47mm caliber proved unsuitable, as its comparatively large and heavy projectile did not allow for high gun mobility or an acceptable rate of fire. These problems were, of course, eventually solved, but this happened only towards the end of World War II.
In search of a solution to increase gun power while simultaneously maintaining a high rate of fire, two countries attempted to solve this problem by introducing intermediate 40mm caliber guns, positioned between the insufficiently powerful 37mm and the excessively heavy 47mm.
One of these countries was Great Britain, with its 2-pounder naval gun (2 PR. No.1). The British, as true conservatives, could not forgive themselves for adopting a metric caliber weapon, and designated it by the approximate projectile weight of two pounds – about 900 grams. This was later followed by the 2 PR. No.2 tank/anti-tank gun, as well as 40mm automatic anti-tank aircraft cannons.
The second country was Sweden with the famous 40mm anti-aircraft gun from Bofors.
The first experience in creating an automatic gun was the Hotchkiss revolving cannon, developed in the 1870s. The design of this weapon likely drew inspiration from the manual crank mechanism of the Gatling gun (though it differed significantly in construction). The 37mm Hotchkiss was quite widely used in various countries, primarily in the navy, but in Great Britain, its characteristics were deemed unsatisfactory. The British took their own path, simply scaling up the famous Maxim machine gun.
The one-pounder Maxim (1 PR. Maxim), which earned the unofficial name “pom-pom” due to its characteristic firing sound (which later became even more well-known and popular than the official designation), with belt feed, used the same low-powered Hotchkiss 37x94R cartridge. Ammunition weight varied, but typically consisted of a 555-gram shell leaving the barrel at an initial velocity of 367 m/s.
The first use of the hypertrophied Maxim came in the last year of the 19th century, and it’s noteworthy that the new weapon was first experienced by the British themselves during the Anglo-Boer War. The gun was subsequently used by British forces but proved of little use. For naval application, it was too underpowered to inflict significant damage on enemy ships, and in the army, its effectiveness was significantly inferior to new shrapnel shells, which were more versatile. In short, at first, the British found nothing appealing about automatic cannons.
World War I, and in particular the advent of aviation, breathed new life into the rapid-fire gun project. It turned out that for combating this new adversary, rapid-fire guns were perfectly suited. It soon became obvious that the low velocity of the 1-pounder shell was a hindrance in combating aircraft, so more powerful versions of the weapon were urgently developed.
The Vickers corporation, which held exclusive rights to Hiram Maxim’s weapons within the British Empire, developed the first 1¼ PDR, and then the second 1½ PDR Mk.I variant of the 37mm naval gun, firing a much more powerful 37x123R ammunition. These, sharing the same casing, differed only in the power of the shell itself. These guns were not particularly widespread, though they were produced in series (the “Vickers 1½ PDR Mk.I Class B” was even experimentally installed a couple of times on British seaplanes), before evolving into the 2-pounder “pom-pom” – the 2 PR. No.1. In this iteration, while retaining the general principle, the casing neck was expanded to 40mm, and the length increased to 158mm, yielding a new cartridge known as 40x158R. The new gun underwent numerous modifications, remaining in service with the British fleet until the 1950s.
The Genesis of the COW Gun
As mentioned above, the appearance of the first airplanes necessitated the development of more powerful means of combat than light machine guns. The 1-pounder Maxim proved too heavy for aircraft installation, so Vickers developed lighter versions of this weapon. In parallel, two variants were developed: the manually loaded 1.2-pounder 40mm “1.59-inch Vickers Q.F.Mk II” (often referred to as “Vickers-Crayford rocket gun”) and the automatic 37mm 1 PR Mk III. Both guns used low-powered cartridges. The first used 40x79R – a shortened version of the naval 40x158R, and the second used the 37x69R cartridge.
The first variant was somewhat more successful; at least it found application. The Vickers Q.F.Mk II gun was originally conceived as a light mobile infantry artillery system, mounted on a light tripod for use in trench warfare. The gun could be operated by one person. The very weak 40x79R cartridge predetermined a very low muzzle velocity for the high-explosive shell – only 244 m/s, or 305 m/s for the armor-piercing round. The gun did not catch on with the infantry.
Its effectiveness left much to be desired, and compared to cheaper, lighter, and more powerful mortars, it offered no particular advantages. Therefore, army commanders quickly tried to get rid of it. In 1917, an attempt was made to adapt the gun for use on aircraft. In April 1917, several FE.2 fighters from 100 and 102 squadrons of the RAF were equipped with Vickers Q.F.Mk II guns and used in night operations as ground-attack aircraft. However, manual loading nullified all the advantages of a powerful weapon for aviation. The gun was also planned for use against German airships, but it seems this remained just an idea.
A more famous automatic cannon adopted by the British Empire was developed by the state arsenal “Coventry Ordnance Works” – its abbreviation “COW” was a constant source of irony, given that “cow” is English for the animal.
Coventry Ordnance Works was established in 1905 as an attempt to create a competitor to the arms monopolists Vickers and Armstrong-Whitworth in the field of naval artillery. Early experiments with installing guns on aircraft showed that single-shot artillery systems were not effective enough, and an attempt was made to create an automatic system. The first variant developed by the company was a 1-pounder – based on the weight of a 37×94 mm rimless shell. The gun was tested on the FE.3 aircraft in 1913. The gun was located in the front cockpit, and firing was conducted only on the ground to assess the impact of recoil forces on the aircraft structure.
Further development led to a more powerful 1.5-pounder variant of the 37mm automatic gun, which used a powerful 37×190 mm shell. The new COW gun was a long-recoil weapon, with its return spring wrapped around the barrel. Ammunition was fed from a 5-round magazine. The small ammunition capacity meant a limited practical rate of fire, although the technical rate of fire was 100-120 rounds/min.
Research and development took a long time and were not completed until August 1917, when the COW Mk.III gun was formally adopted. The first order was for 90 units of this weapon – the maximum that could be produced without detrimental effects on other gun production. By August 1918, the order was increased to 450 units and received high priority, despite the gun’s reliability issues. By the end of hostilities, only 14 COW guns had been delivered to the troops, and the total production volume was only 76 guns.
In October 1918, the COW gun was mounted in the rear cockpit of three light DH-4 bombers, at a 45º angle, for combating airships and giant German “R” bombers like the Rohrbach. A special embrasure for the gun barrel was made in the upper wing of the biplane box. The second crew member served as the gunner/loader.
To protect the light wood and fabric structure of the DH-4 aircraft from the impact of hot powder gases, certain parts of the upper wing and fuselage were shielded with additional metal screens. During tests, over 1000 rounds were fired from the gun on the ground and in the air. Experiments were conducted with various weapon mounting angles, eventually settling on an 85° angle. In November 1918, two of the three DH-4 gun-fighters were sent to the front, but they did not manage to participate in combat before the armistice.
Until the end of World War I, the 1½ Pr. COW Mk.III gun was experimentally installed on a number of British aircraft. These included the “Tellier” flying boat; Voisin 8; DH-3A; DH-10 and FE-4 bombers, as well as defensive armament on the heavy Handley-Page V/1500 bomber. After the war ended, experiments with the gun’s installation continued. In 1921, the 37mm COW gun was installed in the front cockpit of Armstrong Whitworth “Sinaia” and Vickers “Valentia” bombers, and the Short “Cromarty” flying boat. These aircraft were intended for ground attack roles. Experiments continued throughout the 1920s.
Design and Operational Principles
Perhaps the most notable and original projects for installing the COW gun on aircraft were the fighters developed under specification F.29/27 in 1931 by Vickers and Westland. These were single-seat, single-engine aircraft essentially built around the gun. No other armament was provided. The gun was angled upwards at 45-55 degrees and intended for attacking bombers from below, thus serving as a kind of great-grandfather to the future German “Schräge Musik” installations.
In both projects, the gun was placed to the right of the pilot, and aiming was done through a Barr & Stroud periscope sight. Firing trials showed that, theoretically, a single shell hit was enough to destroy the target. Due to recoil, the sight would be knocked off target, requiring the pilot to re-aim the aircraft. The “artillery” trigger mechanism proved to be quite stiff, requiring considerable effort. Ammunition on the experimental aircraft consisted of 5-6 five-round clips. Reloading the gun in the air was envisioned to be performed by the pilot.
The experimental fighters were tested in 1931-32, but the client quickly lost interest in the gun-fighter project. After these experiments, the COW gun project was essentially closed. Some of them were used in the late 1930s as stationary anti-aircraft guns for the air defense of British airfields. Some were used on trucks as mobile self-propelled units at the beginning of World War II, though they most likely never saw combat. In 1948, the last examples of the “COWs” were sent for smelting as finally obsolete.
The total production volume of the COW gun was 450 units. Interestingly, 50 of them were sold to France in the early 1920s, though their traces are lost there. In the interwar years, Coventry Ordnance Works worked on more powerful systems of similar design, but with calibers of 47mm and even 75mm, yet these projects did not move beyond the prototype stage.
The COW gun’s operating principle was a classic example of a weapon working on the basis of a long recoil barrel, meaning their travel exceeded the cartridge length. The barrel and breech were locked together throughout their backward movement under the effect of the firing recoil. In general terms, the principle of operation resembled a scaled-up Chauchat machine gun.
On one hand, the long recoil stroke implied a low rate of fire, but on the other hand, it was significantly more reliable. It automatically solved the problem of needing to open the breech while it was under high gas pressure, unlike shorter recoil systems.
The gun featured air cooling for the barrel. As noted above, it was fed from 5-round clips. To initiate firing, the shooter pushed the first round into the chamber using the bolt handle moving forward, locking the breech with a subsequent turn of the handle.
Pressing the trigger fired the round. Powder gases forced the barrel with the locked breech backward. After the barrel and breech traveled a distance exceeding the cartridge length, the breech unlocked, the casing was extracted, after which the barrel returned to its forward position, and the breech was held by the sear. Then, if the trigger was still pressed, the breech moved forward, chambering the next round, and the cycle repeated.